1. Field of the Invention
The present invention relates to the use of organic ligands to stabilize nanocrystal colloids in solution. More specifically, the present invention relates to the use of ligands, especially cross linked ligands, to prevent oxidation, deterioration, precipitation and segregation of nanocrystals in a colloidal suspension or solid matrix.
2. Prior Art
Nanotechnology is one of the fastest growing fields in the industry. Microscopic devices have countless applications. Currently, the chemistry behind synthesis and processing of nanomaterials are the two key areas of research scientists are focusing on. Once nanomaterials can be readily synthesized and processed, almost any device could be formed on a nanoscopic scale.
Colloidal nanocrystals are nanometer sized fragments of corresponding bulk crystals dispersed in solvents or other types of matrices. They are one of the many materials being explored for a variety of applications because of their novel, size dependent properties and potentially flexible processing chemistry. In some cases, the nanometer sized particles may not be crystalline materials. Nanocrystals and nanoparticles show promise for use as light emitting diodes (LEDs), labels for bioassays and a host of other applications. Recent progress in the synthesis of high quality of nanocrystals, especially chalcogenides nanocrystals for use as semiconductors, as highlighted the need for methods for maintaining and handling these nanocrystals. Adequate methods of processing nanocrystals are essential for utilizing them in nanodevices and other nanostructures. For example, the size dependent emission properties of semiconductor nanocrystals make them highly desirable as labeling reagents for biomedical applications and as color tunable emitting materials in LEDs and lasers.
Compared to conventional photoluminescence-based labeling reagents, semiconductor nanocrystals possess many invaluable advantages. With their very narrow but size-dependent band-edge emission spectrum and their extremely broad absorption spectrum, semiconductor nanocrystals simplify the current detection scheme significantly. The emission of semiconductor nanocrystals can last magnitudes longer than that of conventional organic dyes under optical radiation. By simply tuning the size of very few kinds of semiconductor nanocrystals, the detection window can cover the same wavelength range of tens of different organic dyes.
In addition, conjugation chemistry for coupling different sizes/types of semiconductor nanocrystals with bio-functional species can potentially be the same because the coupling reactions always occur on the outer surface of the ligand layer. This has already been shows with cadmium selenide nanocrystals having a zinc sulfite shell and a thiol-silica ligand layer. Unfortunately, this process is very difficult to reproduce and the resulting nanocrystals deteriorate rapidly and are not well suited for subsequent purification procedures.
Ligands on the surface of colloidal nanocrystals, sometimes also called capping groups or surfactants, etc., are certain types of organic molecules with two distinguishable parts. One part of the molecule can chemically bind to the surface of the inorganic nanocrystals (or nanoparticles), and the other part helps the nanocrystals (or nanoparticles) to be dispersible in solvents or matrices.
Nanocrystals often precipitate out of solution because of the loss of their thiol ligands. One method of alleviating this problem is to use di-thiol ligands to increase stability. Unfortunately, the process is still difficult and the nanocrystals still deteriorate relatively rapidly.
One of the significant problems with nanocrystals is their tendency to rapidly oxidize and loose their unique properties. Another difficulty with nanocrystals is their low solubility. Die to their size, they must be held in a colloidal suspension, and are by their very nature insoluble. The use of thiol ligands has been an attempt to solve both of these problems.
By attaching ligands to the surface of the nanocrystals, it has been hoped that this would sterically hinder oxidation. While ligands do have a slight impact on oxidation, they have to date been unable to give nanocrystals the stability necessary for desired applications.
Similarly, ligands have so far proved disappointing in being able to maintain a colloidal suspension of nanocrystals. Functional groups having desired hydrophobic or hydrophilic properties have been attached to the ends of the ligands and do in fact aid to maintain a colloidal suspension. However, because the ligands dissociate, the colloids have a relatively short life time.
It is therefore desirable to provide a method of maintaining a colloidal suspension of nanocrystals for a very long time.
It is also desirable to provide a method for preventing oxidation of nanocrystals.